1. Field of the Invention
The present invention relates to a method and an apparatus for deaerating liquids, especially water, in order to remove a substantial portion of the oxygen that has been absorbed by the liquid.
2. The Prior Art
It is known to inject water, e.g., seawater into subterranean hydrocarbon-bearing strata in order to displace the hydrocarbons present so as to achieve a better recovery of the hydrocarbon resource.
Injection of seawater containing absorbed oxygen, however, can cause certain disadvantages. The included oxygen will, for example promote corrosion of the utilized piping and, in addition, will, result in an undesired growth of aerobes which are conveyed by the injected water. Growth of aerobionts in the hydrocarbon bearing formation can result in colonies of the microorganisms which will act to seal off the hydrocarbon-bearing stratum, such that the desired recovery thereof cannot be achieved.
Oxygen and other gases can be desorbed or removed from water by boiling the water at atmospheric pressure, but this method is unsuitable for treatment of large quantities of water. Absorbed gases can also be removed by gas stripping, which can be carried out in a gas stripper degasser. Such a gas stripper degasser is usually built as a counter current stripping column containing a packing or perforated trays. Water is introduced at the top of the degasser and the stripping gas, e.g., purified natural gas, is introduced at the bottom of the column and bubbles up through the water in intimate contact therewith, the stripping gas displacing other absorbed gases, so that water at least partly freed from, e.g., oxygen is obtained.
Gas-stripping, however, has certain disadvantages, i.e., the utilized equipment is expensive, heavy and bulky, which is most disadvantageous on an offshore rig where it occupies space and represents an undesired weight.
a large amount of natural gas that is poor in sulphur is consumed and must be burned after use; and there is a gas hazard in the area around such a plant which results in restrictions as to the positioning of the plant as well as necessitating various safety measures.
Because the amount of absorbed gas in a liquid is proportional to the partial pressure of the gas above the liquid, absorbed gases can also be removed by reducing the partial pressure of the gas above the liquid. For this purpose, vacuum deaerators of a conventional kind, being constructed similar to the gas stripping degassers, can be used, but instead of introducing gas at the bottom, the upper part of the deaerator is preferably connected to a suitable vacuum pump for reduction of the partial gas pressure. In connection with the conventional vacuum deaerators, the desired effect necessitates a multi-stage deaerator. Such a vacuum deaerator is, thus, as heavy and as bulky as a gas-stripper degasser. Also, in the final step it is often necessary to add oxygen-consuming chemicals to achieve acceptable low oxygen values. An addition of chemicals requires further expensive dosing and control equipment and the cost of chemicals will further increase overheads.
The known technology of the art has been described by Dr. Charles C. Patton in "Oilfield Water Systems" (Petro Tech Ltd. 1977). The mentioned paper contains a description of today's equipment and methods as well as a copious list of references to further literature.
The present invention relates to an improved method and apparatus for vacuum deaerating of liquids, especially water, permitting one-step deaeration which results in very low values of residual free oxygen in the treated water. The improved method also permits use of substantially less bulky equipment than the known methods.